Heat pumps are thermodynamic devices that can move thermal energy from a first temperature source to a second, higher temperature sink. This transfer of thermal energy in a direction opposite to the direction it passively flows (i.e., it passively flows from a higher temperature to a lower temperature) requires the expenditure of energy which can be supplied to the heat pump in various forms including electricity, chemical energy, mechanical work or high grade thermal energy.
During warm weather heat pumps are commonly used to move thermal energy from within a building to ambient, i.e., they provide comfort air conditioning to the occupied spaces within buildings. This air conditioning has two important components: sensible cooling, which reduces the temperature within the building, and latent cooling, which reduces the humidity. Comfortable and healthy indoor conditions are maintained only when both the indoor temperature and humidity are controlled, and so a heat pump's sensible and latent cooling are both important.
Unfortunately, heat pumps are not efficient latent cooling devices. Since they “pump” thermal energy and no moisture, they dehumidify only when the process air is cooled below its initial dewpoint temperature. In many applications, the process air that is cooled to a low temperature so that water vapor condenses must be reheated so that a comfortable indoor temperature is maintained. This process of overcooling and reheating wastes energy and increases the cost to maintain comfortable indoor conditions.
Desiccant air conditioners can be a more efficient means for controlling indoor humidity. Desiccants are materials with a high affinity for water vapor. They can be used to directly absorb water vapor from air without first cooling the air below its dewpoint temperature. After the desiccant absorbs water vapor it is heated so that the absorbed water vapor is released to an appropriate sink (e.g., the outdoor ambient). This release of water vapor regenerates the desiccant to a state where it can then again absorb water vapor.
In one type of desiccant air conditioner, the thermal energy for regenerating the desiccant is supplied by the refrigerant condenser of a vapor-compression heat pump. The following five patents and patent applications describe different ways to implement a liquid-desiccant air conditioner that regenerates the desiccant with thermal energy recovered from a refrigerant condenser:
Peterson, et al., U.S. Pat. No. 4,941,324
The Peterson patent describes a vapor-compression air conditioner in which the external surfaces of both the evaporator and condenser of the air conditioner are wetted with a liquid desiccant. Both water vapor and heat are absorbed from the process air that flows over the desiccant-wetted surfaces of the evaporator. The desiccant rejects water to a stream of cooling air that flows over the desiccant-wetted surfaces of the condenser. Under steady operating conditions, the concentration of the desiccant naturally seeks a value at which the rate water is absorbed by the desiccant on the evaporator equals the rate water is desorbed by the desiccant on the condenser.
Forkosh, et al., U.S. Pat. No. 6,546,746; Griffiths, U.S. Pat. No. 4,259,849
Both the Forkosh patent and Griffiths patent describe a vapor-compression air conditioner in which a liquid desiccant is cooled in a refrigerant evaporator and heated in a refrigerant condenser. The cooled desiccant is delivered to and spread over a first bed of porous contact media. Process air that flows through this first porous bed is cooled and dried. The heated desiccant is delivered to and spread over a second bed of porous contact media. Cooling air that flows through this second porous bed gains thermal energy and water vapor from the warm liquid desiccant. As with the Petersen patent, under steady operating conditions the concentration of the desiccant naturally seeks a value at which the rate water is absorbed by the desiccant on the evaporator side of the heat pump equals the rate water is desorbed by the desiccant on the condenser side.
Vandermeulen, et al., U.S. Patent Application US 2012/0125020
The Vandermeulen patent application describes a vapor-compression air conditioner in which a first heat transfer fluid is cooled in a refrigerant evaporator and a second heat transfer fluid is heated in a refrigerant condenser. The cooled first heat transfer fluid cools a first set of membrane-covered plates that have a liquid desiccant flowing on the surface of each plate under the membrane. Process air is cooled and dried as it flows in the gaps between the first set of plates in contact with the membranes. The heated second heat transfer fluid heats a second set of membrane-covered plates that have a liquid desiccant flowing on the surface of each plate under the membrane. The cooling air gains thermal energy and water vapor from the desiccant as it flows in the gaps between the second set of plates in contact with the membranes. As with the Petersen patent, under steady operating conditions the concentration of the desiccant naturally seeks a value at which the rate water is absorbed by the desiccant on the evaporator side of the heat pump equals the rate water is desorbed by the desiccant on the condenser side.
Dinnage, et al., U.S. Pat. No. 7,047,751
The Dinnage patent describes a vapor-compression air conditioner in which the cool, saturated process air that leaves the refrigerant evaporator of the air conditioner flows through the first of two sectors of a desiccant wheel, and the warm, unsaturated cooling air that leaves the refrigerant condenser of the air conditioner flows through the second sector. Water vapor is absorbed from the process air by the desiccant in the first sector and desorbed to the cooling air by the desiccant in the second sector. The desiccant wheel rotates between the two air streams so that absorption and desorption processes occur simultaneously and continuously.
A fifth patent by Lowenstein, et al., (U.S. Pat. No. 7,269,966) describes a technology to implement a liquid-desiccant air conditioner functionally similar to that described in the Peterson patent when the liquid desiccant is a corrosive halide salt solution.
Heat pumps that augment their latent cooling using technology described in the either the Griffiths, Forkosh, Vandermeulen or Dinnage patents will all have fundamental performance limitations. Because the Griffiths and Forkosh patents use beds of porous contact media that are adiabatic (i.e., there is no embedded, internal source of cooling or heating within the beds) desiccant flooding rates must be high compared to the flow of air through the beds. These high flooding rates are required so that the desiccant's temperature neither increases significantly (in the bed where heat is released as the desiccant absorbs water) nor decreases significantly (in the bed where heat is absorbed as the desiccant desorbs water). These high flooding rates require large pumps with high power draws. They also produce large air-side pressure drops in the flooded beds that increase the heat pump's fan power.
A heat pump that uses the Vandermeulen technology must pump a cooling heat transfer fluid between its thermal sink (e.g., a refrigerant evaporator for a heat pump that uses vapor-compression technology) and the liquid-desiccant absorber and it must pump a heating heat transfer fluid between its thermal source (e.g., a refrigerant condenser for a heat pump that uses vapor-compression technology) and the liquid-desiccant desorber. These two heat transfer loops both increase the heat pump's power use and degrade performance by introducing temperature drops that force the heat pump's thermal sink to run at a lower temperature and its thermal source to run at a higher temperature.
The source of the limitations inherent in a heat pump that uses the Dinnage technology is the solid desiccant rotor. In particular:                (a) There is no simple way to pre-cool the warm regeneration (i.e., water desorption) sector of the desiccant wheel as it rotates into the air stream that is to be dehumidified. The heat stored in the mass of the wheel is therefore transferred to this air stream, thereby reducing the cooling effect provided by the air conditioner. Similarly, a significant fraction of the thermal energy in the warm air that regenerates the solid desiccant performs the task of heating the mass of the wheel as the cool process (i.e. water absorption) sector of the solid desiccant wheel rotates into the warm air stream. This heating task reduces the quantity of thermal energy in the warm air that actively desorbs water from the desiccant.        (b) The regeneration sector and process sector of the desiccant wheel must be next to each other. This geometrical constraint requires that the supply air and the regeneration air flow counter to each other in very close proximity.        (c) The circular shapes of the regeneration sector and process sector differ from the rectangular shape that is common for the finned-tube heat exchangers that serve as the air conditioner's refrigerant evaporator and refrigerant condenser. Whereas design constraints on either the height or width of an air conditioner can be accommodated by adjusting the aspect ratio of a rectangular heat exchanger, the desiccant wheel must grow (or shrink) by the same proportion in both its height and width.        
A heat pump that applies the technology in the Lowenstein patent also has important limitations, although the limitations are not fundamental, rather centering on the practical concerns of the investment in capital equipment required to manufacture a new heat pump design. In particular, when implemented as a vapor-compression air conditioner the technology in the Lowenstein patent would require a manufacturer to use radically different assembly procedures for the air conditioner's evaporator and condenser then are now used for conventional finned-tube heat exchangers.